feat: add vitest for testing and implement tests for calculateDay utility

- Added "test" script to package.json for running vitest.
- Updated api.js to use localhost for backend during development.
- Created utils.spec.js to test calculateDay function with various scenarios.
- Updated vite.config.js to include vitest configuration and conditionally load Vue DevTools.

frontend/vite.config.js

@@ -1,16 +1,20 @@
+/// <reference types="vitest" />
 import { fileURLToPath, URL } from 'node:url'
-
+import VueDevTools from 'vite-plugin-vue-devtools'
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
-import vueDevTools from 'vite-plugin-vue-devtools'
 
-// https://vite.dev/config/
-export default defineConfig({
+// https://vitejs.dev/config/
+export default defineConfig(({ command }) => ({
   base: '/odoo/',
-  plugins: [vue(), vueDevTools()],
+  plugins: [vue(), ...(command === 'serve' ? [VueDevTools()] : [])],
   resolve: {
     alias: {
       '@': fileURLToPath(new URL('./src', import.meta.url)),
     },
   },
-})
+  test: {
+    globals: true,
+    environment: 'jsdom',
+  },
+}))